1. Field of the Invention
The invention is directed to a method for operating a hearing aid device or hearing device system having at least one first microphone for generating a first microphone signal and a second microphone distanced from the first for generating a second microphone signal. The invention is also directed to a hearing aid device or hearing device system for implementing the method.
2. Description of the Related Art
Acoustic feedback frequently occurs in hearing aid devices, particularly for hearing aid devices having a high gain. This feedback is expressed in pronounced feedback-caused oscillations having a specific frequency, called “whistling”, that is usually extremely unpleasant both for the hearing aid user and as well as for people in the immediate surroundings.
Feedback can occur when sound that is picked up via the microphone of the hearing aid device is amplified by a signal amplifier and output via the earphone proceeds back to the microphone and is re-amplified. In order for the typical “whistling” to occur (usually at a dominant frequency), however, two further conditions must be met. First, the “loop amplification” of the system, i.e., the product of the hearing aid gain and the attenuation of the feedback path, must be greater than 1. Second, the phase shift of this loop amplification must correspond to an arbitrary whole-numbered multiple of 360°.
The simplest approach for reducing feedback-conditioned oscillations is to permanently reduce the hearing aid gain so that the loop amplification remains below the critical limit value even in unfavorable situations. The critical disadvantage of this approach, however, is that the hearing aid gain required given more pronounced hearing impairment can no longer be achieved as a result of this limitation.
Other approaches provide a measurement of the loop amplification during the hearing aid adaptation and reduce the hearing aid gain in designational fashion in the critical range with the assistance of what are referred to as notch filters (narrow-band blocking filters). Since the loop amplification, however, can constantly change during daily life, the benefit is likewise limited.
A number of adaptive algorithms have been proposed for dynamic reduction of feedback-conditioned oscillations, these automatically adjusting to the respective feedback situation and implementing corresponding measures. These methods can be roughly divided into two categories.
The first category comprises “compensation algorithms” that estimate the feedback part in the microphone signal with the assistance of adaptive filters and neutralize the feedback by subtraction and, thus, do not deteriorate the hearing aid gain. However, these compensation methods assume uncorrelated, i.e., ideally, “white”, input signals. Tonal input signals that always exhibit a high time correlation lead to an incorrect estimate of the feedback path, which can lead to the fact that the tonal input signal itself is erroneously subtracted.
The second class contains the algorithms that only become active when feedback-conditioned oscillations are present. They generally contain a mechanism for detecting oscillations that continuously monitors the microphone signal. When feedback-typical oscillations are detected, the hearing aid gain is reduced to such an extent that the loop amplification drops below the critical limit. The gain reduction can ensue, for example, by lowering a frequency channel or by activating a suitable narrow-band stop filter (notch filter). This is disadvantageous because the oscillation detectors can fundamentally not distinguish between tonal input signals and feedback whistling. The result is that tonal input signals are interpreted as feedback oscillations and are then incorrectly reduced in level due to the reduction mechanism (for example, notch filters).
Delay elements that have a decorrelating effect are often introduced into the signal processing chain in the compensation algorithms in order to prevent tonal signal segments having a length characteristic for voice signals from being noticeably attacked. Due to echo effects and irritations as a result of desynchronized visual and auditive information, however, only delays in the range milliseconds are allowed. For example, the reduction of music to signals, which are often correlated over a clearly longer time span, cannot be avoided. A further counter-measure is comprised in retarding the adaptation of the filter so long that all relevant tonal useful signals are not attacked. However, this also results in the compensation filter no longer being able to follow rapid changes of the feedback path quickly enough, so that feedback-conditioned oscillations arise for a certain time and in turn disappear only when the feedback path has stabilized and the filter has again adequately adapted. The negative consequences of incorrect detection of oscillation detectors are countered in that the resulting reduction in gain occurs to only a limited extent, so that tonal useful signals erroneously considered to be feedback-conditioned oscillations (for example, alarm signals) still remain audible. This, however, harbors the risk that the reduction of gain in the feedback case does not suffice in order to fall below the critical limit and thus eliminate the “whistling”.
In summary, the functioning of all adaptive feedback-reduction methods is deteriorated by input signals that exhibit a tonal character affected by dominant sine signal parts (for example, triangle tones, alarm signals). This often leads to unacceptable tonal deteriorations of the input signal.
German patent document DE 693 27 992 T2 discloses a feedback-suppression arrangement with adaptive filtering for a hearing prosthesis that comprises two microphones in a specific embodiment. It does not implement a detection of oscillations.
U.S. Pat. No. 6,072,884 A discloses a device for suppressing feedbacks that likewise comprises two microphones. It does not implement a detection or a comparison of oscillations.
German patent document DE 199 22 133 A1 discloses a hearing aid device with an oscillation detector. This device comprises only one microphone, so that a comparison of a plurality of microphone signals is not possible.